Many therapeutically active molecules do not possess the properties required to achieve efficacy in clinical medical use. For example, therapeutically active proteins and polypeptides are now being discovered and produced by the biopharmaceutical industry and by genetic engineering. Although there are currently at least 80 protein based medicines marketed in the United States with at least 350 more protein based medicines undergoing clinical trails (Harris J, Chess R: Effect of Pegylation on pharmaceuticals. Nature Review Drug Discovery, 2003, 2, 214-221), most native proteins do not make good medicines because upon administration to patients there are several inherent drawbacks that include: (1) proteins are digested by many endo and exopeptidases present in blood or tissue, (2) many proteins are immunogenic to some extent and (3) proteins can be rapidly excreted by kidney ultrafiltration. Other molecules used as active therapeutic agents in medicines that are systemically toxic or lack optimal bioavailability and pharmacokinetics include low molecular weight molecules where an effective dose is limited by toxicity. Such molecules are routinely used to treat inflammation and conditions due to malignancies, infection and autoimmune disease.
Water soluble, synthetic polymers, particularly polyalkylene glycols, are used to conjugate therapeutically active molecules such as proteins. These therapeutic conjugates have been shown to favourably alter pharmacokinetics by prolonging circulation time and decreasing clearance rates, decrease systemic toxicity, and in several cases, to display increased clinical efficacy. This process of covalently conjugating polyethylene glycol, PEG, to proteins is commonly known as “PEGylation” however many different polymers have been examined as conjugating polymers.
Many polymer reagents for conjugation comprise conjugating chemical functionality that is hydrolytically unstable. Examples of hydrolytically unstable polymeric conjugating reagents are active esters that include, for example, polyalkylene oxide-N-succinimide carbonates (Zalipsky U.S. Pat. No. 5,122,614). These reagents have relatively short half lives in aqueous media, that includes blood or plasma. This results in the need to add large stoichiometric excesses of the conjugating polymer reagent. The hydrolytic stability of the reagent is important because the requirement to add stoichiometric excesses for protein conjugation requires significant effort and cost to purify the polymer-protein conjugate from the reaction mixture. Furthermore, these hydrolytically unstable reagents tend to undergo preferentially, reaction with amine chemical functionality in the protein, particularly to the ε-amine of lysine residues. Since most proteins of interest have more than one lysine residue, and frequently many lysine residues, then conjugation tends to be non-specific in that it occurs at many residue sites on the protein. It is possible to purify the conjugating reaction mixture to isolate proteins conjugated to one polymer molecule, however it is not possible, to isolate at a reasonable cost, polymer-protein conjugates that are all conjugated to the same amine group on the protein. Non-specific conjugation frequently results in impaired protein function. For example antibodies and antibody fragments with random poly(alkylene oxide) attachment via lysine residues result in modified antibodies (or modified antibody fragments) able to bind target antigen with reduced affinity, avidity or specificity. Additionally, amine specific polymer conjugating reagents require conjugating reaction conditions that must be selected to ensure that the amines on the protein are not protonated. These conditions require moderately high pH media (8-10), this allows the amine moieties to be reactive enough for reaction with the polymer conjugating reagent. High pH conditions are frequently deleterious to the protein causing structural changes and denaturation. These processes result in impairment of protein function. Amine specific polymer conjugation reagents tend to bind to accessible amine sites on the protein. These reagents can be termed kinetic reagents. They are labile and undergo a reaction with the most assessable amino nucleophilic sites on the protein. Amine specific polymer conjugating reagents that conjugate by amine acylation result in the loss of positive charge on the amine group of the amino acid residue on the protein that would normally be present under physiological conditions for the unconjugated protein. These features of amine specific polymer conjugating reagents often leads to partial impairment of the function of the protein. Other conjugating functional groups incorporated in polymers for conjugation to protein and that are amine specific and frequently hydrolytically labile include isocyanate (WO 94/04193) and carbonates (WO 90/13540).
Particularly relevant for optimised efficacy and to ensure dose to dose consistency is to make certain that the number of conjugated polymer molecules per protein is the same and that each polymer molecule is specifically covalently conjugated to the same amino acid residue in each protein molecule. Non-specific conjugation at sites along a protein molecule results in a distribution of conjugation products and frequently, unconjugated protein, to give a complex mixture that is difficult, tedious, and expensive to purify.
Thiol specific polymer conjugating reagents for proteins have been developed to address the limitations for the propensity of the conjugating reagent to undergo hydrolysis that is competitive with conjugation to the protein, non-specific polymer conjugation at different amino acid residues in the protein, and the need for high pH conjugating reaction conditions. Thiol specific polymer conjugating reagents can be utilised at pH values close to neutrality where the amine functional moieties on the amino acid residues of the protein are protonated and thus cannot effectively compete in the conjugation reaction with the polymer conjugating reagent. Thiol specific polymer conjugating reagents that are relatively more hydrolytically stable than are the aforementioned amine specific reagents can be utilised at a lower stoichiometric excess thus reducing the cost during purification of the polymer-protein conjugate. Conjugating functional moieties that are broadly selective for thiol groups include iodoace-tamide, maleiimide (WO 92/16221), vinylsulfone (WO 95/13312 and WO 95/34326), vinyl pyridines (WO 88/05433), and acrylate and methacrylate esters (WO 99/01469). These thiol selective conjugating moieties yield a single thioether conjugating bond between the polymer.
Most proteins do not have free sulfhydrals because these sulfhydrals undergo rearrangement and scrambling reactions with the disulfide bridges within the protein resulting in impaired protein function. For proteins that do have free sulfhydrals, these sulfhydrals are frequently critical for protein function. Typically in a protein, the number of sulfhydral moieties is less than the number of amine moieties (e.g. lysine or histadine). Since conjugation to a protein can be made to be specific at thiol groups and since proteins do not typically have free thiol groups, there are examples of site-specific modification of protein by mutagenesis to introduce thiol sites for PEG attachment. However such modifications increase costs significantly. The introduced free sulhydral can have the similar limitations as mentioned heretofore in the engineered protein for protein scrambling and protein dimerisation. Also the process of mutagensis and production of the modified protein from bacterial sources frequently causes the free sulfhydral to be bound in a disulfide bond with glutathione, for example. Interleukin-2, for example, has been modified by mutagenesis to replace a threonine residue by a cysteine to allow site specific attachment of PEG. [Goodson R J, Katre N V; Bio/Technology (1990) 8, 343-346].
It is known in the art that conjugating parameters have to be optimally matched with the therapeutically active molecule of interest in terms of polymer morphology, molecular weight characteristics, chemical functionality. Although the polymer protein conjugate, can display many favourable and necessary properties needed for safe, effective medical use, the effect of polymer conjugation on the activity and stability of the protein is of vital importance for performance. Conjugation variables related to the location and amount of conjugation and polymer characteristics must be optimally correlated with biological and physicochemical properties.